Vibration Monitoring of Induced Draft (ID) Fan


Large Fans and Blowers (ID, FD, CF, PA etc.) are classified under ISO 10816-3 (Large Machinery) and are used in process plants for moving air or gases. They are essential for cooling, heating or transporting product. They come in many designs but usually are centrifugal or axial driven. A common feature is that over time, erosion and/or corrosion and/or deposition build up on the blades occurs. This results in a progressive increase in vibrations due to unbalance of the rotor. If ignored, consequential damage to the drive motor’s bearings and coupling will result.

Not always are these faults progressive and predictive. An even build-up of dirt or cake on a fan’s impeller is quite normal but, should a piece become detached then suddenly the rotor becomes out of balance. This can be extremely destructive and threaten personnel and plant safety. The loss of a single line Fan or Blower can shut down the process causing significant production and financial losses.

A continuous vibration monitoring system will give timely warning to prevent premature plant failure. Fans or Blowers power consumption often account for over 12% of the total electricity used and it is a well recorded fact that an out of balance fan will consume or waste up to 15% more energy

An Induced Draft (ID) Fan
Points where Vibration should be measured on a Motor Fan set


Due to its operational application; analysing and designing a Vibration Monitoring System must take into account the following:

  • Unbalance
  • Misalignment
  • Bad bearings
  • Mechanical looseness
  • Aerodynamic forces

Typical Vibration Sensor Measurement Positions

Locating the vibration sensors as indicated in the schematic will be most sensitive and effective in detecting the above faults listed. The sensor orientations are summarised as follows:

  1. HORIZONTAL on two motor bearings (A & B) and two fan bearings (C&D). Measures velocity mm/sec (Peak or RMS) to detect unbalance and structural rigidity & foundation problems.
  2. VERTICAL on motor & fan drive end bearings (B&C). Measures velocity mm/sec (Peak or RMS) to detect looseness and structural rigidity & foundation problems.
  3. AXIAL on motor & fan drive end bearings (B&C). Measures velocity mm/sec (Peak or RMS) to detect misalignment between motor & fan (E).

Vibration Diagnostic Chart - Faults common to Fans and Blowers

 1 Unbalance Proportional to unbalance in radial direction 1 x RPM Single steady reference mark Most common cause of vibration. Correct by balancing each rotating parts before assembly then after assembly
2 Misalignment of couplings, bearings or bent shaft Axial vibration 50% of more of the radial level1 x RPM common but often x 2 or 3 RPMSingle double or triple reference mark Best identified by dominant axial vibration. Confirm with phase measurement or dial gauges. If sleeve bearing with no coupling misalignment balance the rotor
 3 Anti-friction Bearings Use Spike Energy® (gSE), DemodulationHigh frequency 35K to 55K CPM not related to RPM.Erratic Sensor proximity to a defective bearing is a clear indicator. External signals like steam leaks and cavitation can give false readings
4  Mechanical LoosenessOften highest in vertical direction 2 x RPM 1 or 2 reference marks depending on frequency. Usually unsteady Usually accompanied by unbalance and/or misalignment
5  Aerodynamic ForcesAxial readings may be higher than normal1 x RPM or number of blades on fan or impeller x RPM Steady if one blade damaged - like unbalance Likely to occur where blade is bent or out of track. Use noise analysis for high frequencies.

Solutions for Online Monitoring

IRD7100 - Machinery Protection Transmitter (MPT)

Provides Instantaneous Amplitude Display out in field, 4-20 mA output, Set-point configurable Alarm and Trip Relay Contacts

IRD8800 - Machinery Protection Monitor API 670

Provides Instantaneous Amplitude Display, 4-20 mA output, MODBUS RS485 RTU/TCP-IP output, Fully Programmable Setpoints and Alarm and Trip Relays

Cloud Based Vibration Monitoring

Hardware located close to Machine. Data pushed to Cloud over 2G/4G/Ethernet. Plots and Reports accessible via any Browser/Cloud Mobile App

If you need help choosing a Solution for your Induced Draft (ID) Fan or any other equipment, feel free to Contact Us. You can also call us on +91-2248972000 (Monday to Saturday, 9:00 AM to 6:00 PM IST)

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